Improving the energy efficiency of building comfort systems has become increasingly more important due to rising energy costs, as well as increased awareness and concern over global warming as a result of humanity's rising consumption of carbon fuels for electrical energy generation, direct burn heating, and domestic hot water appliances. One area where these concerns can be addressed is through improving the efficiency of solar-based HVAC systems by generating ice during hours of sunshine for later use during the night or cloud cover when solar radiation is inadequate. In traditional HVAC system this can also be beneficial through leveling demand by shifting some of the load during peak hours of a day to off-peak times, thereby eliminating the need to build and run expensive and inefficient peak generator turbines (peakers).
Demand control and increased efficiency is primarily accomplished by shifting the burden of cooling from the hottest time of the day to the nighttime when ambient temperatures, as well as demand, are considerably lower. Refrigeration equipment efficiency increases when the temperature lift requirement decreases. The difference in temperature lift between a hot day and a cool night can often be as high as 50%, thereby resulting in a massive drop in refrigeration equipment lift requirements and a corresponding efficiency increase. The demand for cooling is usually highest during peak hours when outside temperatures and solar radiation are at their highest levels which results in increased electrical consumption. In order to prevent strain on the power grid, utilities are often forced to use gas turbine peak generators for only a few of the hottest hours of the year. The efficiency of these generators is typically 40 to 50% lower than steam turbines which generate most of our electricity. An alternative to peak generation is Thermal Energy Storage (TES) technology.
While there are different types of thermal storage systems on the market the most common designs are based on cold water or two-phase ice/water storage. In recent years the ice storage systems have increased in popularity due to a considerably higher energy storage density. Currently ice storage systems are commonly used in large buildings and campuses. These systems will generally contain chillers which cool a secondary heat transfer media (such as an ethylene-glycol solution) to below the freezing point of water and circulate it through the heat exchangers of ice storage tanks.
Ice storage tanks are usually comprised of rectangular or cylindrical water-filled vessels containing heat exchangers. The heat exchangers are primarily made of circular copper or plastic tubing. The cooling solution flows through the heat exchanger thereby freezing the water. Examples of such systems are disclosed in U.S. Pat. No. 4,831,831 to Carter et al. and U.S. Pat. No. 6,247,522 to Kaplan et al.
These types of systems have several shortcomings. First they occupy a considerable amount of floor space for the chiller and the ice storage tanks Secondly, the solutions used as the heat transfer media are generally expensive, toxic, and have inferior heat transfer properties to water which increases the required pumping energy. And finally, the ratio of ice volume to the full volume of the storage tank is not very high due to the heat exchanger coil occupying a considerable amount of the tank's volume.
The process of calculating the growth of freezing water around multiple tubes is complicated and costly thereby making it impractical for commercial markets. The heat exchanger design is usually accomplished through an experimental approach which is expensive, time consuming and rarely produces satisfactory results. Pockets of water can be encapsulated by ice, then, when these pockets freeze, expansion can generate very high pressures which can damage the tubes and/or the shell. This problem is generally solved by restricting the entire tank water volume from freezing solid which in turn further reduces the average ice storage density and increases the size and weight of the tank required for meeting the cooling demand.
Another example of an approach used for ice-based thermal energy storage systems is disclosed in U.S. Pat. No. 7,124,594 to McRell. The thermal energy storage apparatus is comprised of a tank filled with water and a heat exchanger consisting of a multitude of spiral copper tubing coils connected to upper and lower headers. During the ice generating mode these coils are filled with liquid refrigerant provided by a condensing unit which evaporates and freezes the surrounding water. During cooling mode the liquid refrigerant is pumped into cooling coils inside the air conditioning equipment where it evaporates and is fed into the ice storage tank coils, surrounded by a slurry of ice and water, and is cooled and condensed back into liquid.
These systems are complicated and expensive. Also the density of ice storage is relatively low due to the fact that some of the water must remain unfrozen to ensure proper water circulation at the beginning of the cooling mode and to prevent coil damage due to the high pressures generated by the expansion of freezing ice.
U.S. Pat. No. 6,079,481 to Lowenstein et al. discloses a thermal energy storage system where the heat exchanger assembly is made of substantially flat profile boards disposed in a rectangular tank filled with water. A cooling medium with a low freezing temperature flows from a chiller through the boards and freezes the water, and then this solution flows through the load and back through the boards thawing the ice. While this design is potentially capable of increasing thermal energy storage density, it still requires separate spaces for the chiller and the thermal storage unit and requires a heat transfer medium with a freezing temperature below that of water.
Medium capacity chillers usually have direct expansion, tubes-in-shell evaporators. The refrigerant flows through the tubes and the water (or another heat transfer medium) circulates through the shell. Refrigerant is injected in the tubes and evaporates to cool the water. Each evaporator is designed for a specific load, so a chiller manufacturer must carry multiple models of evaporators with a wide range of capacities. Another shortcoming of tubes-in-shell evaporators is the necessity to prevent the heat transfer fluid from freezing on the tubes which would lead to reduction of their heat transfer properties and even their damage.